1. Field of Invention
This invention relates to flow measurement and particularly to in vivo blood flow measurement. As used herein flow is the measurement of fluid movement expressed in terms of a product of fluid density and fluid velocity. Blood flow measurement is an important diagnostic technique used in various medical applications. A prime application is in the diagnostic analysis and treatment of heart conditions, where accurate flow rate measurement can be extremely useful.
2. Description of the Prior Art
Various techniques have been developed for measuring flow, and specifically measuring the volume flux or average flow rate of blood as an incompressible fluid in a vascular system. One such technique is an indicator dilution technique wherein a sterile marking dye is injected, either as an impulse or continuously, in exact measurement into a cardiovascular system and then the concentration is sensed at a downstream measuring point. A flow rate can be computed based on conservation of mass principles. Such a technique however requires a number of assumptions regarding the system, and the technique can be very time consuming.
A thermodilution technique is known for measuring flow which employs an instrumented catheter inserted through the area where flow is to be measured. A four-lumen catheter developed by Swan & Ganz employs the injection of a quantity of heat, such as a hot or cold saline solution, through the catheter upstream of the measuring point. Change in temperature downstream of the measuring point is measured by a thermometer. Flow is computed by analysis of the change in temperature with time. As with the dye injection technique, the Swan-Ganz technique employs injections of a sterile foreign substance and is inherently intermittent in its operation.
It is known that fluid flow can be calculated in a closed system from a dispersion function. The dispersion function is generally an impulse response function obtained as a result of the administration of an indicator by means of an impulse injection. Other deterministic approaches, such as step response measurement and sinusoidal response measurement have also been employed to calculate fluid flow.
Both of the above prior art techniques rely on conservation of mass principles. All of these techniques are basically deterministic in nature and rely on assumptions which may not necessarily be true. For example, the assumption is made that the flow of the indicator is representative of the flow of the total fluid and that the distribution of transit times between the point of origin and the point of measurement is the same for elements of both the total fluid and of the indicator. Moreover, it is assumed that the indicator control volume is constant between a single inlet port and a single outlet port, and that there is no recirculation from the outlet port to the inlet port during the period of measurement. Furthermore, assumptions are made that the mean transit time can be calculated by the mean arrival time of the volume of indicator at the point of measurement.
Techniques are known in the signal processing art for measuring electronic system impulse response by application of nondeterministic signal excitation. Such techniques are heretofore used generally only in communications applications.
What is needed is a flow measuring technique, and particularly a blood flow measuring technique, which is substantially immune to uncertainties associated with deterministic measuring techniques.